Process for flocculating suspensions

ABSTRACT

A process of flocculating and dewatering an aqueous suspension of suspended solids comprising, introducing into the suspension, (a) a concentrated polymer solution and, (b) a dilute polymer solution, characterised in that the concentrated and dilute polymer solutions are introduced into the substrate substantially simultaneously. Preferably the concentrated and dilute polymer solutions are introduced into the suspension as an aqueous composition comprising a dilute aqueous solution of polymer and a concentrated solution of polymer. The process brings about improvements in filtration and cake solids.

This invention relates to processes of flocculating aqueous suspensionsin order to effect separation of solids from said suspension.

It is well known to apply polymeric flocculants to aqueous suspensionsin order to separate solids from the suspension. For instance it iscommon practice to flocculate and then dewater suspensions containingeither suspended solid organic material or mineral solids. For instanceit is common practice to flocculate sludges such as sewage sludge, wastewaters, textile industry effluents, red mud from the Bayer Aluminaprocess and suspensions of coal tailings etc. Flocculants are alsocommonly used in paper-making processes by addition of polymericflocculants to the cellulosic suspension. Flocculation is usuallyachieved by mixing into the suspension polymeric flocculant, allowingthe suspended particles to flocculate and then dewatering theflocculated suspension. In papermaking this removal of water from thecellulosic suspension is often referred to as draining.

High molecular weight polymeric flocculants are commonly used for thispurpose. High molecular weight flocculants may be cationic, anionic,nonionic or amphoteric in nature. The choice of polymeric flocculantwill largely depend upon the susbstrate, which is being treated. Forinstance it is common practice to use high molecular weight cationicflocculants to treat aqueous suspensions comprising suspended organicmaterial, for instance sewage sludge. In paper-making it is known to useeither cationic, nonionic, anionic or amphoteric flocculants.Flocculation of mineral suspensions is frequently effected by use ofanionic flocculants.

It is also known to use two different polymeric flocculants in the sameprocess. The flocculants may have the same charge (co-ionic). Forinstance in commercial practice in the dewatering sewage sludge thesemay be co-ionic. In other processes it is known to apply two polymers ofopposite charge (counter-ionic). Where two polymeric flocculants areapplied to an aqueous suspension they may be added to simultaneously ormore usually sequentially.

It is standard practice to apply polymers as aqueous solutions toflocculate suspensions containing suspended organic material. Generallythe solutions of polymers are relatively dilute, for instance below0.5%, often below 0.3% and usually 0.1% to below 0.2% by weight.

Polymers are usually provided as a solid particulate product or as areverse phase dispersion or emulsion. It is standard practice todissolve the polymer into water by dispersing the polymer particles in aflowing stream of water in the case of the solid particulate product orin the case of the emulsion or dispersion, inversion into water, by useof activator surfactants. The polymer solution thus formed is frequentlyat a concentration above 0.3%, often in the range 0.4% to 1% and usuallyabout 0.5%. This more concentrated solution of polymer may be tooconcentrated to add directly to the suspension in many instances, sincereceived wisdom suggests that there would be inadequate distribution ofthe flocculant throughout the suspension and as a consequence theflocculation process would be impaired.

It is therefore common practice to first of all provide a moreconcentrated solution of polymer and then dilute the polymer solutionprior to application. Often the diluted solution will have aconcentration of below 0.2%, for instance within the range 0.05 to 0.2%by weight and frequently between 0.1 and 0.2% by weight. This dilutesolution of polymer is normally metered directly into the suspensionprior to the dewatering stage.

There is a desire to improve the efficiency of the flocculationprocesses, to either bring about an increased dewatering effect, such ashigher cake solids or in the alternative achieve a constant acceptablelevel of dewatering efficiency but using a lower dose of flocculant.This is true in a variety of flocculation processes, includingdewatering of sewage sludge, slurries of coal tailings, red mud and inpapermaking.

It would therefore be desirable to provide an improved method offlocculating and dewatering aqueous suspensions of solids, in particularto provide increased dryness of the dewatered solids for an equivalentdose of flocculant or to provide the same degree of dryness of dewateredsolids but using a reduced dose of flocculant. It would also bedesirable to provide a process which provides faster dewatering.

The invention relates to a process of flocculating and dewatering anaqueous suspension of suspended solids comprising, introducing into thesuspension,

(a) a concentrated polymer solution and,

(b) a dilute polymer solution,

characterised in that the concentrated and dilute polymer solutions areintroduced into the substrate substantially simultaneously.

The concentrated and dilute solutions may be metered directly into thesuspension as separate solutions. By substantially simultaneously thetwo solutions should be added at approximately the same dosing point.Where the concentrated and dilute solutions are added to the suspensionseparately, they may be added in either order. For instance if thedilute solution is added first the concentrated polymer may be addedafter flocculation has commenced but should be added before thedewatering stage and before any high shear stage, such as pumping orscreening stages. Alternatively, it may be desirable to add the dilutepolymer solution after the addition of the concentrated polymersolution. When the dilute and concentrated polymer solutions are addedseparately it may be appropriate to allow or apply some degree mixingbetween the dosing stages in order to allow the first polymer dose tobecome distributed throughout the suspension solids. This mixing may forinstance include allowing the treated suspension to pass some distancealong a flow line which optionally contains bends, baffles,constrictions or other features which induce gentle mixing.

Preferably the concentrated and dilute polymer solutions are introducedsimultaneously.

More preferably the concentrated and dilute polymer solutions areintroduced into the suspension as an aqueous composition comprising adilute aqueous solution of polymer and a concentrated solution ofpolymer. The aqueous composition should comprise both the dilute and theconcentrated polymer solutions as discrete components. Thus it isdesired that the dilute solution and concentrated solution exist assubstantially discrete components of the aqueous composition.

The aqueous composition preferably comprises the dilute aqueous solutionof polymer in an amount of from 20 to 99%, based on weight of polymer,and the concentrated polymer solution in an amount of from 1 to 80%,based on weight of polymer. For some applications, such as for rotaryvacuum filtration coal tailings slurries it may be appropriate to use aratio of concentrated solution to dilute polymer solution of around75:25. However in most other applications the ratio of concentratedpolymer solution to dilute polymer solution would generally in the range1:99 to 40:60.

The aqueous composition comprising concentrated and dilute solutions maybe of any significantly different concentrations provided that therespective concentrations are not substantially the same such that thetwo solutions would immediately form a homogenous single solution.Preferably the concentrated solution should be at least twice theconcentration of the diluted solution. More preferably the concentratedsolution should be at least 4 or 5 times the concentration of the diluteaqueous solution.

The dilute aqueous solution of polymer desirably has a concentration ofpolymer of below 0.5%, preferably below 0.3% by weight. More preferablythe concentration of the dilute solution is in the range 0.05 to 0.2%,most preferably around 0.1% by weight.

According to the invention the polymer dissolved in the dilute aqueouspolymer solution may be either cationic, anionic or non-ionic.

The concentrated aqueous solution component according to the inventiondesirably has a concentration of polymer above 0.3% by weight,preferably between 0.4 and 1.0% by weight. More preferably theconcentration of the concentrated solution is in the range 0.5 to 1.0%.According to the invention the polymer dissolved in the concentratedaqueous polymer solution may be either cationic, anionic or non-ionic.The polymer dissolved in the concentrated polymer solution is preferablyeither co-ionic with the polymer dissolved in the dilute solution ornon-ionic. In another preferred form the polymer dissolved in the dilutesolution is non-ionic and the polymer dissolved in the concentratedpolymer solution is cationic, anionic or non-ionic.

When the polymer dissolved in either the dilute solution or concentratedsolution is cationic, said cationic polymer may be formed bypolymerisation of at least one cationic monomer alone or with othermonomers. Suitable cationic monomers include quaternary ammonium or acidsalts of monomers which contain amine groups. Preferably the cationicpolymer is formed from a monomer or blend of monomers comprising atleast one cationic monomer selected from the group consisting ofquaternary ammonium and acid salts of dimethylaminoethyl (meth)acrylate, quaternary ammonium and acid salts of dimethylaminoethyl(meth) acrylamide and diallyldimethyl ammonium chloride. The cationicmonomers may be hompolymerised or copolymerised with other monomers, forinstance acrylamide. In addition to vinyl addition polymers, thecationic polymer may include polymers obtained by condensation oraddition reactions. For instance suitable cationic polymers includeadducts of amines with epihalohydrins or dihaloalkanes, polyamides andpolyethylene.

In the case where the polymer dissolved in either the dilute solution orconcentrated solution is anionic, said anionic polymer may be formed bypolymerisation of at least one anionic monomer alone or with othermonomers. Suitable anionic monomers include ethylenically unsaturatedmonomers comprising carboxylic acid or sulphonic acid groups. Preferablythe anionic polymer is formed from a monomer or blend of monomerscomprising at least one anionic monomer selected from the groupconsisting of (meth) acrylic acid, 2-acrylamido-2-methylpropanesulphonic acid, alkali metal and ammonium salts thereof.

If the polymer dissolved in either the dilute solution or concentratedsolution is nonionic, said anionic polymer may be formed bypolymerisation of suitable non-ionic monomers, for instance acrylamideor methacrylamide.

The polymers suitable for both the concentrated aqueous solution anddilute aqueous solution may be prepared by any convenient polymerisationprocess, for instance gel polymerisation, reverse phase suspensionpolymerisation, reverse phase emulsion polymerisation, solutionpolymerisation and the like. Thus suitable polymers may be provided inthe form of granulated powders, beads, reverse phase emulsions, reversephase dispersions or aqueous solutions.

The concentrated aqueous solution may be formed by dissolving anysuitable water soluble polymer into water. The dilute aqueous solutionof polymer may also be prepared by dissolving any suitable water solublepolymer into water or alternatively by diluting a more concentratedsolution of the polymer solution. The respective concentrated and diluteaqueous solutions may be produced therefrom by known dissolution,inversion or dilution techniques as appropriate. For instance solidparticulate cationic polymer may be dissolved by dispersing the polymerparticles into a flowing stream of water. Reverse phase emulsions orreverse phase dispersions of cationic polymers may be inverted intowater by use of activator surfactants to form the respective aqueoussolutions. Preferably the polymers dissolved in both the dilute andconcentrated solutions are essentially the same polymer.

The polymers according to the invention may be prepared as substantiallylinear polymers or as branched or structured polymers. Structured orbranched polymers are usually prepared by inclusion of polyethylenicallyunsaturated monomers, such as methylene-bis-acrylamide into the monomermix, for instance as given in EP-B-202780. Preferably however, thepolymers are substantially linear and are prepared in the form of a beador powdered product.

A particularly preferred group of polymers includes copolymers ofacrylamide with at least one cationic monomer selected from the groupconsisting of quaternary ammonium and acid salts of dimethylaminoethyl(meth) acrylate, quaternary ammonium and acid salts ofdimethylaminoethyl (meth) acrylamide and diallyldimethyl ammoniumchloride, having an intrinsic viscosity of at least 4 dl/g. The cationicacrylamide polymers may comprise 10 to 90% by weight acrylamide and 10to 90% by weight cationic monomer(s).

The aqueous composition comprising the dilute aqueous solution ofcationic polymer and the concentrated solution of cationic polymer maybe formed by introducing the concentrated solution of cationic polymerinto a flowing stream of the dilute aqueous solution of cationicpolymer. For instance in one method of preparing the aqueous compositiona concentrated aqueous solution of cationic polymer is introduceddirectly into a conduit through which the dilute aqueous solution ofcationic polymer is being conveyed towards the dosing point where theaqueous composition comprising both concentrations of polymer aremetered into the suspension of solids in order to effect flocculation.

According to a preferred form of the invention wherein an aqueoussuspension of suspended solids is flocculated and dewatered, an aqueouscomposition is introduced into the suspension. The aqueous compositioncomprises concentrated and dilute aqueous solutions of cationic polymerwherein the two solutions exist as discrete components of thecomposition. It is considered desirable that the mixture of concentratedand dilute solutions exist together as a non-homogenous composition.Therefore, in order to prevent the concentrated solution fromdissipating and being diluted thus forming a homogenous solution ofpolymer at a single concentration, it is desirable to substantiallyreduce any mixing of the aqueous composition prior to being introducedinto the suspension. One way that undesirable mixing of the aqueouscomposition can be avoided is by ensuring that there are no mixing orpumping stages after the concentrated and dilute solutions have beencombined. In addition it may further be desirable for the conduit tohave a relatively smooth inner surface and the avoidance short radiusbends, for example as given in pending International Application No.PCT/GB 99/00990. Another way that undesirable mixing can be avoided isto reduce the distance the aqueous composition has to travel bycombining the concentrated and dilute solutions relatively close to thedosing point.

It is desirable that the aqueous composition comprising dilute andconcentrated solutions does not contain substantial amounts ofundissolved polymer, for instance it is preferable that less than 5%,more preferably less than 2% by weight of total polymer contained in theaqueous composition is not in solution. In many dewatering situationsthe most efficient use of the polymer is achieved if the amount ofundissolved polymer is less than 1%, especially less than 0.5%.

The dilute solution of cationic polymer may conveniently be prepared bydilution of a more concentrated solution of the polymer. This can beachieved by adding dilution water to a flowing stream of moreconcentrated solution of polymer. For instance it may be desirable topass the more concentrated solution of cationic polymer along a conduitto a dilution stage, where dilution water is introduced into theconcentrated solution. In order to achieve adequate mixing of theconcentraed solution with the water so that a homogenous consistentdilute solution is obtained it may be necessary to introduce a mixingstage. The mixing stage may be for instance an in-line mixing stage,such as an in-line static mixer, a pumping stage, a screening stage orsome other means that can ensure adequate mixing. Preferably oncethoroughly mixed the diluted solution will be substantially homogenous.

A particularly preferred aspect of the invention relates to a process offlocculating and dewatering an aqueous suspension of suspended solids byintroducing into the suspension an aqueous composition comprisingconcentrated and dilute polymer solutions wherein the aqueouscomposition is formed by,

(a) passing a concentrated solution of polymer to a dilution stage wherethe solution is combined with dilution water to form a dilute solution,

(b) passing the diluted solution through a mixing stage, selected froman in-line mixer, a pumping stage and screening stage, and

(c) introducing a concentrated solution of polymer into the diluteaqueous solution.

The concentrated polymer solution, which is diluted to form the dilutepolymer solution may be drawn from the same reservoir of concentratedpolymer solution which is subsequently combined with the dilute solutionin forming the said aqueous composition according to the invention.There may be some mixing of the dilute and concentrated polymersolutions provided that this does not result in the aqueous compositionbecoming substantially homogenous.

Thus in a particularly preferred process for preparing the aqueouscomposition a concentrated aqueous solution of cationic polymercontained in a holding vessel is passed through a conduit to a dilutionstage and subsequent mixing stage thus providing the dilute aqueoussolution. Concentrated aqueous solution of cationic polymer contained inthe holding vessel from said holding vessel is passed by means of asecond conduit directly into the dilute aqueous solution of cationicpolymer. A typical arrangement for carrying out the preparation of theaqueous composition according to this aspect of the invention is shownin FIG. 1.

In FIG. 1 the following key applies,

[1] Holding vessel containing concentrated cationic polymer solution

[2] Conduit conveying concentrated cationic polymer solution to dilutionstage

[3] Dilution water line

[4] Pump

[5] Conduit conveying dilute cationic polymer solution

[6] Conduit conveying concentrated polymer solution

[7] Aqueous composition comprising concentrated and dilute aqueoussolutions of cationic polymer

[8] Sewage sludge line

[9] Dewatering stage

[10] Dosing point of aqueous composition into the sludge

[11] Dilution stage

[12] Pump

Thus in the scheme represented in FIG. 1, aqueous concentrated cationicpolymer solution is held in holding vessel [1]. Concentrated polymersolution is passed along conduit [2] towards dilution stage [11] afterwhich the aqueous polymer solution and dilution water passed through apump [4] where they are mixed together to ensure that a consistentdiluted polymer solution is formed. The dilute aqueous polymer solutionis passed along conduit [5] towards the point where concentrated polymersolution is added. A second conduit [6] from holding vessel [1] conveysconcentrated cationic polymer solution into the dilute polymer solutionto form the aqueous composition [7] which is passed to the dosing point[10] where the mixture of concentrated and dilute cationic polymersolutions are metered into the sewage sludge line [8]. The treatedsewage sludge is then passed into the dewatering stage [9].

Alternatively the concentrated polymer solution which is combined withthe dilute aqueous polymer solution may be drawn from a separatereservoir of concentrated polymer solution from that which is diluted toform the dilute aqueous polymer solution. Thus in this alternative formof the invention the opportunity exists for the concentrated polymerbeing a different polymer from the polymer in the dilute aqueoussolution. For instance it may be desirable to combine a concentratedsolution of a low molecular weight cationic polymer, having an intrinsicviscosity of below 3 dl/g, with a dilute solution of a high molecularweight cationic polymer, having an intrinsic viscosity of at least 4dl/g. The low molecular weight polymer may be a coagulant, for instancethe hompolymer of diallyldimethyl ammonium chloride. The high molecularweight polymer may be a bridging flocculant, for example a copolymer ofacrylamide with a suitable cationic monomer, such as the quaternaryammonium salt of dimethylaminoethyl (meth) acrylate. A typicalarrangement for conducting this alternative aspect of the invention isshown in FIG. 2.

In FIG. 2 the following key applies, [1] Holding vessel containingconcentrated cationic polymer solution [2] Conduit conveyingconcentrated cationic polymer solution to dilution stage [3] Dilutionwater line [4] Pump [5] Conduit conveying dilute cationic polymersolution [6] Second holding vessel for concentrated cationic polymersolution [7] Conduit conveying concentrated polymer solution [8] Aqueouscomposition comprising concentrated and dilute aqueous solutions ofcationic polymer [9] Sewage sludge line [10] Dewatering stage [11]Dosing point of aqueous composition into the sludge [12] Pump [13]Dilution stage

Thus in the scheme represented in FIG. 2, aqueous concentrated cationicpolymer solution is held in holding vessel [1]. Concentrated polymersolution is passed along conduit [2] towards dilution stage [13] afterwhich the aqueous polymer solution and dilution water are passed througha pump [4] where they are mixed together to ensure that a consistentdiluted polymer solution is formed. The dilute aqueous polymer solutionis passed along conduit [5] towards the point where concentrated polymersolution is added. A second conduit [7] passes concentrated aqueouscationic polymer solution from holding vessel [6] into the dilutepolymer solution to form the aqueous composition [8] which is passed tothe dosing point [11] where the mixture of concentrated and dilutedcationic polymer solutions are metered into the sewage sludge line [9].The treated sewage sludge is then passed into the dewatering stage [10].

The invention is suited to a variety of processes involving flocculationand dewatering. Processes of particular relevance include dewateringsewage sludges, dewatering mineral suspensions, dewatering of paper millsludges, dewatering of deinked cellulosic sludges e.g. from paperdeinking plants and also papermaking processes.

The following examples serve to illustrate the invention.

EXAMPLE 1

Aqueous solutions of a copolymer of acrylamide with dimethylaminoethylacrylate, methyl chloride quaternary ammonium (40/60 weight/weight),intrinsic viscosity at least 10dl/g, are prepared at 0.1, 0.125 and 0.5%concentration. Composition 1 is prepared by introducing a 0.1% solutioninto 0.5% solution on a 50/50 weight/weight basis. Composition 2 isprepared in a similar manner to composition 1 by combining a 0.1%solution with 0.5% solution on a 75/25 weight/weight basis.

200 ml aliquots of Rotherham (Yorkshire, England) sewage sludge aretreated with dilute polymer (0.1%) and (0.125%), concentrated polymer(0.5%) and using composition 1 and composition 2 each at various dosesof cationic polymer. The treated sludge is mixed at 2000 rpm for 15seconds. The flocculation efficiency is measured by free drainage usinga 10 cm diameter sieve.

The free drainage results are shown in Table 1.

TABLE 1 5 second filtrate volume (ml) for each dose Polymer solution137.5 mg/l 150 mg/l 162.5 mg/l 0.1% 10.5 31 55 0.125% 4 24 50 0.5% — 2749 Composition 1 19 41 79 Composition 2 14 32 67

The results clearly show the advantage of using the compositionscomprising a mixture of concentrated and dilute solutions of thecationic polymer.

EXAMPLE 2

Example 1 is repeated, except using polymer solutions 0.1%, 0.167% and0.5% and mixed composition of 0.1% and 0.5% (50/50) and using 250 mlaliquots of Rotherham sewage sludge and subjecting the treated sludge tomixing at 7000 rpm for 15 seconds. The flocculation efficiency ismeasured by free drainage using a 8 cm diameter sieve. For each test thevolume of filtrate is measured and adjusted to allow for the volume ofeach aqueous polymer dose.

The adjusted free drainage results are shown in Table 2.

TABLE 2 Polymer 5 second filtrate volume (ml) for each dose solution 100mg/l 120 mg/l 140 mg/l 160 mg/l 180 mg/l 0.1% 73 116 159 166 149 0.167%71 114 163 174 165 0.5% 79 124 165 176 165 (50/50) mix- 83 166 167 166155 ture of 0.1% and 0.5% polymer solutions

The results clearly show that optimum drainage is achieved using a lowerdose of the blend of 0.1% and 0.5% polymer solutions than any of theother treatments.

EXAMPLE 3

Example 2 is repeated using a copolymer of acrylamide withdimethylaminoethyl acrylate, methyl chloride quaternary ammonium (80/20weight/weight), intrinsic viscosity at least 10 dl/g prepared as areverse phase emulsion, which has been dehydrated to form a liquiddispersion product and inverted in water to form aqueous solutions ofthe polymers at various concentrations. These polymer solutions aretested using 500 ml aliquots of Rotherham sewage sludge which has beendiluted with water (2 parts sludge to 3 parts water) and subjecting thetreated sludge to mixing at 1000 rpm (low shear) for 15 seconds. Theflocculation efficiency is measured by free drainage using a 8 cmdiameter sieve. The adjusted free drainage results are shown in Table 3.

TABLE 3 5 second filtrate volume (ml) for each dose 30 40 50 60 70 80Polymer solution mg/l mg/l mg/l mg/l mg/l mg/l 0.1% 175 181 246 290 296270 0.167% 121 158 246 302 308 256 0.5% 157 206 256 314 303 262 (50/50)mixture of the 0.1% 131 158 285 322 308 256 and 0.5% polymer solutions

The results clearly demonstrates that the blend of polymer solutionsgive an increased optimum drainage by comparison to the othertreatments. This is visible from the plot of these results shown in FIG.3.

EXAMPLE 4

Example 3 is repeated except the polymer has been prepared according tothe teaching of EP-A-202780 by including about 20 ppmmethylenebisacrylamide with the monomer resulting in a cross-linkedpolymer, which exhibits an ionic regain of 40%. The treatment is asdescribed in example 3, except the treated sludge is subjected to mixingat 4,000 rpm the flocculation efficiency is measured by free drainageusing an 8 cm sieve.

The free drainage results adjusted for dose volume are shown in Table 4.

TABLE 4 5 second filtrate volume (ml) for each dose 90 100 110 120 130140 Polymer solution mg/l mg/l mg/l mg/l mg/l mg/l 0.1% 195 260 345 350345 0.167% 223 320 347 364 361 347 0.5% 221 320 369 370 382 376 (50/50)mixture of the 0.1% 303 360 387 393 370 and 0.5% polymer solutions

The results clearly demonstrate that the blend of polymer solutions andseparate and sequential treatment of different concentrations exhibit onthe whole improved drainage by comparison to the other treatments. Aplot of these results is shown in FIG. 4.

EXAMPLE 5

Example 3 is repeated except the treatment comprising the mixture of0.1% and 0.5% polymer solutions is replaced by sequential dosing of the0.1% and 0.5% polymer solutions, wherein the 0.1% solution is addedfirst, followed by mixing for 5 seconds at 4,000 rpm and then applyingthe 0.5% polymer solution, followed by further mixing for 15 seconds at4,000 rpm and then draining through an 8 cm sieve.

The free drainage results adjusted for dose volume are shown in table 5

TABLE 5 5 second filtrate volume (ml) for each dose (Total Polymer Dose)70 80 90 100 110 120 Polymer solution mg/l mg/l mg/l mg/l mg/l mg/l0.05% 80 130 210 260 300 280 0.1% 125 230 265 320 325 310 0.167% 119 226293 320 337 324 0.2% 113 200 288 335 342 320 0.3% 108 197 275 333 352330 0.4% 111 210 289 347 356 335 0.5% 103 172 281 340 339 328 sequentialdosing of 0.1% 129 246 314 330 331 and 0.5% polymer solutions

The results clearly demonstrate that effective dewatering of the sludgecan be achieved using lower total polymer dose by applying dilute andconcentrated polymer solutions by comparison to the other treatmentsemploying single concentration polymer solutions. Thus the mixedconcentration dosing enables more efficient dosing of the polymer.

EXAMPLE 6

Aqueous solutions of a copolymer of acrylamide with dimethylaminoethylacrylate, methyl chloride quaternary ammonium (75/25 weight/weight),intrinsic viscosity at least 10 dl/g, are prepared at 0.1, 0.125 and0.5% concentration. A mixture of 0.1% and 0.5% solution as a weightratio of 75:25 is also prepared.

Dewatering of a de-inked paper mill sludge (0.91% solids) was evaluatedusing the polymer solutions at various doses. For each test the polymerwas dosed to 600 ml of sludge, followed by stirring for 15 seconds at2000 rpm using a 4 blade stirrer. The flocculation efficiency wasmeasured using free drainage through an 8 cm sieve recording filtratevolume after 5 seconds. The free drainage results, adjusted to take intoaccount the dose volumes are shown in table 6.

TABLE 6 5 second filtrate volume (ml) for each dose Polymer solution 1Kg/t 2 Kg/t 4 Kg/t 0.1% 125.5 139 108 0.125% 145.6 141.2 82.4 0.5% 148.9147.8 115.6 75:25 mixture of 0.1% solution and 0.5% 145.6 161.2 102.4solution

As can be seen by the results of this test the mixture of dilute andconcentrated polymers solutions provides improved optimum free drainage.

EXAMPLE 7

Example 6 was repeated except instead of measuring free drainage, thetreated sludge was transferred to a piston press. Pressures of 20, 40,60 and 80 pounds per square inch (psi) were applied in 2 minuteincrements.

The cake produced was then weighed wet and dry to calculate the cakesolids.

The results are shown in table 7.

TABLE 7 Cake Solids % Polymer solution 1 Kg/t 2 Kg/t 4 Kg/t 0.1% 25.1526.94 30.83 0.125% 31.82 29.84 33.09 0.5% 42.93 26.56 31.24 75:25mixture of 0.1% solution and 0.5% 26.34 32.31 32.95 solution

The results from examples 6 and 7 show that the mixed dilute andconcentrated polymer solutions provide the best overall combination offree drainage and cake solids.

EXAMPLE 8

A suspension of China Clay is prepared and used 4% (weight/volume) in 2g/l sodium chloride solution. The tests are carried out on 500 mlaliquots of the China Clay suspension and mixed with various doses ofpolymer solutions of specified concentrations at 500 rpm impeller speed.The duration of mixing is for 15 seconds for single doses andsimultaneous doses.

The flocculated China Clay is for each test transferred to a 500 mlmeasuring cylinder immediately upon completing the mixing stage. Thetime taken for the solid liquid interface (mud line) to pass between the3 cm and 8 cm level is measured. A sedimentation rate in cm/minute iscalculated and shown for each total polymer dose in table 8.

In each test the polymer is a copolymer of acrylamide with sodiumacrylate, with a monomer ratio by weight of 70:30.

TABLE 8 Sedimentation rate (cm/min) Polymer solution 3 mg/l 4 mg/l 5mg/l 0.05% 21 40.7 57.1 0.0833% 15 41.2 55.6 0.25% 14.8 36.5 43.8 50:50mixture of 0.05% solution and 27.2 55.9 83.6 0.25% solution

The results show that the mixed concentrated and dilute polymersolutions provide the best sedimentation rates. This is clearly shown inFIG. 5.

EXAMPLE 9

Example 8 is repeated comparing two stage addition of dilute andconcentrated polymer solutions with simultaneous addition, two stageaddition of two dilute solutions and single stage addition of a dilutesolution.

The duration of mixing is and for 15 seconds for single doses andsimultaneous doses and for the two stage dosing of concentrated anddilute polymer solutions, the first dose is applied followed by mixingfor 5 seconds followed by the second dose and then mixing for a further15 seconds.

A sedimentation rate in cm/minute is calculated and shown for each totalpolymer dose in table 9.

TABLE 9 Sedimentation rate (cm/min) Polymer solution 3 mg/l 4 mg/l 5mg/l 6 mg/l 0.05% 10.2 14.6 20.7 38 0.05% two stage addition 12.7 2029.2 42.1 0.05%:0.25% (50:50) two stage 20.2 33.4 38.5 47.1 addition50:50 mixture of 0.05% solution 15.7 32.1 43.7 45.2 and 0.25% solution

The results show that the mixed concentrated and dilute polymersolutions and two stage addition of dilute and concentrated polymersolutions out perform the single dose of dilute polymer solution or thetwo stage dose of dilute polymer solution. This is clearly visible fromthe plots shown in FIG. 6.

What is claimed is:
 1. A process of flocculating and dewatering anaqueous suspension of suspended solids comprising, introducing into thesuspension an aqueous composition comprising, (a) a concentrated aqueouspolymeric flocculant solution having a concentration above about 0.3% byweight and (b) a dilute aqueous polymeric flocculant solution having aconcentration below about 0.3% by weight, characterised in that theconcentrated and dilute polymeric flocculant solutions are introducedinto the suspension substantially simultaneously and the dilute solutionand concentrated solution exist as substantially discrete components ofa non-homogeneous composition, to flocculate said suspended solids, anddewatering said aqueous suspension.
 2. A process according to claim 1 inwhich the aqueous composition comprises, (a) 1 to 80%, by weight of theconcentrated aqueous solution of polymeric flocculant and (b) 20 to 99%,by weight of the dilute aqueous solution of polymeric flocculant.
 3. Aprocess according to claim 1 in which the aqueous composition comprises,(a) 1 to 40%, by weight of the concentrated aqueous solution ofpolymeric flocculant and (b) 60 to 99%, by weight of the dilute aqueoussolution of polymeric flocculant.
 4. A process according to claim 1 inwhich the dilute aqueous polymeric flocculant solution comprises acationic polymer, an anionic polymer or a nonionic polymer.
 5. A processaccording to claim 1 in which the concentrated aqueous polymericflocculant solution has a concentration of polymeric flocculant ofbetween 0.4 and 1.0%, by weight.
 6. A process according to claim 1 inwhich the concentrated aqueous polymeric flocculant solution comprises acationic polymer, an anionic polymer or a nonionic polymer.
 7. A processaccording to claim 1 in which the polymeric flocculant dissolved in theconcentrated solution is either co-ionic with the polymeric flocculantdissolved in the dilute solution or non-ionic.
 8. A process according toclaim 1 in which polymeric flocculant dissolved in either the dilutesolution or the concentrated solution Is cationic and has been formedfrom a monomer or blend of monomers comprising at least one cationicmonomer selected from the group consisting of quaternary ammonium andacid salts of dimethylaminoethyl (moth) acrylate, quaternary ammoniumand acid salts of dimethylaminoethyl (meth) acrylamide anddiallyldimethyl ammonium chloride.
 9. A process according to claim 1 inwhich the polymeric flocculant dissolved in either the dilute solutionor the concentrated solution is anionic and has been formed from amonomer or blend of monomers comprising at least one anionic monomerselected from the group consisting of (meth) acrylic acid,2-acrylamido-2-methylpropane sulphonic acid, alkali metal and ammoniumsalts thereof.
 10. A process according to claim 1 in which the polymericflocculant dissolved in either the dilute solution or the concentratedsolution is nonionic and has been formed from acrylamide ormethacrylamide.
 11. A process according to claim 1 in which thepolymeric flocculant dissolved in each of the dilute and concentratedaqueous solutions is a cationic copolymer of acrylamide and at least onecationic monomer selected from the group consisting of quaternaryammonium and acid salts of dimethylaminoethyl (meth) acrylate,quaternary ammonium and acid salts of dimethylaminoethyl (meth)acrylamide and diallyldimethyl ammonium chloride, having an intrinsicviscosity of at least 4 dl/g.
 12. A process according to claim 1 inwhich the aqueous composition comprising the dilute aqueous solution ofpolymeric flocculant and the concentrated solution of polymericflocculant is formed by introducing the concentrated solution ofpolymeric flocculant into a flowing stream of the dilute aqueoussolution of polymeric flocculant.
 13. A process according to claim 12 inwhich the dilute aqueous solution of polymeric flocculant is formed bydiluting a flowing stream of the concentrated aqueous solution ofpolymeric flocculant with dilution water.
 14. A process according toclaim 1 in which the aqueous composition is formed by, (a) passing saidconcentrated solution of polymeric flocculant to a dilution where thesolution is combined with dilution water to form a dilute solution, (b)passing the diluted solution through a mixing stage, selected frompumping and screening stages, and (c) introducing said concentratedsolution of polymeric flocculant into the dilute aqueous solution toform said aqueous composition.
 15. A process according to claim 14 inwhich the concentrated polymeric flocculant solution, which is dilutedto form the dilute polymeric flocculant solution in step (a) is drawnfrom the same reservoir of concentrated polymeric flocculant solutionintroduced into the dilute solution in step (c).
 16. A process accordingto claim 14 in which the concentrated solution of polymeric flocculantin step (a) is drawn from a different reservoir of concentratedpolymeric flocculant solution introduced into the dilute solution instep (c).
 17. A process according to claim 1 in which the dewateringprocess is selected from the group consisting of dewatering sewagesludge, dewatering a mineral suspension, dewatering a paper mill sludge,dewatering a deinked cellulosic sludge and a papermaking process.